Conventionally, there is known an illumination system in which a CCD (Charged-Coupled Device) camera is used as a sensor for sensing brightness (see, e.g., Japanese Patent Application Publication No. 2004-501496, Paragraph [0016] and FIG. 3). In this illumination system, the brightness of an image pickup area is calculated by averaging the brightness of light detected by pixels in the image data taken by a CCD camera. Based on the results of this calculation, the dimming level of an illumination device is increased or decreased to thereby keep the illuminance of a target space at a substantially constant level.
In the above-described illumination system, the detected brightness would be increased if the reflectance of the target space is changed, e.g., by a white paper sheet placed on a desk. Thus, it may be difficult to keep the illuminance on the desk surface at a constant level. Further, pixels whose brightness exceeds a specified threshold are all excluded from the pixels to be employed in the brightness detection. This makes it impossible to exclude only the pixel area having a change in brightness (or reflectance). As a result, it is sometimes difficult to accurately detect the brightness of the target space.